Interleukin 6 (IL-6) is a well known cytokine, characterized for its essential function in inflammation. IL-6 receptor (IL-6R) is composed of IL-6Rα, an unsignalling chain, associated with the signaling transducing chain gp130. This glycoprotein is shared by all IL-6 family cytokines. After binding with its receptor, IL-6 preferentially induces the activation of the Jak/STAT3 pathway but can also activate the Jak/STAT1 pathway. Unexpectedly we demonstrated that IL-6 can activate the Jak/STAT5 pathway in CD4 T cells. This STAT5 could act as negative feedback mechanism in response to the pro-inflammatory effects induced by an excess of IL-6.
Ciliary neurotrophic factor (CNTF) and cardiotrophin-like cytokine (CLC/CLF) both belong to the IL-6 cytokine family and share the same receptor, the CNTF receptor (CNTFR). CNTFR is composed of CNTFRα, leukaemia inhibitory factor receptor β (LIFRβ) and the glycoprotein gp130. Interestingly, the CNTFRα chain is not expressed by immune cells even though CNTF and CLC/CLF are active on these cells. These effects can be due to the formation of a complex between cytokine and CNTFRα, which can be shedded. This complex can then activate cells expressing only gp130 and LIFRβ. In human, it has been demonstrated that the CNTFRα chain can be substitute with IL-6Rα. Here, we compare mouse CNTF- and CLC/CLF-induced effects in transfected cells expressing LIFRβ, gp130 and different α chains belonging to the IL-6 family (IL-6Rα, IL-11Rα or CNTFRα). Our data demonstrate that like human CNTF, mouse CNTF is able to activate a receptor comprising of IL-6Rα, gp130 and LIFRβ. However, this property is not shared with CLC/CLF. Therefore, second receptor for this cytokine within the immune system still remains to be identify.
Interleukin 27 (IL-27) belongs to the IL-6 cytokine family and is composed of the cytokine subunit p28 associated with a soluble receptor chain Epstein-Barr virus-induced gene 3 (EBI3). We demonstrate that the p28 subunit can bind the soluble receptor CLF to form a new dimeric cytokine named p28/CLF. This cytokine is active on T cells and our study demonstrates its activity on B cells. Our results show that p28/CLF sustains plasma cell differentiation. Those IL-6-like properties can be explained by the use of a common receptor, IL-6R. Moreover, our findings demonstrate that p28 has IL-6-like properties when associated with IL-6Rα. In order to better characterize p28/CLF, we next studied effects of to the recruitment of the IL-6Rα chain by p28 subunit.
Cytokines belonging to the IL-6 family share a structural particularity by forming a four helix bundle cytokines family. The p28 subunit uniquely expresses a motif composed of a dozen of glutamic acids (polyE motif). We demonstrate that this motif permits p28 binding to hydroxyapatite and bone matrix. This observation could allow a preferential targeting to bone of IL-27 (p28/EBI3) and p28/CLF, and specifically a targeting of stem or immune cells to endosteal niches.